PTSD and depression are serious psychiatric disorders afflicting the veteran population, and are frequently comorbid. Current pharmaco- and psychotherapies are inadequate, and more effective treatments are needed. Progress has been hampered by a lack of understanding of the neurobiological mechanisms underlying the pathology and effective treatment of these disorders. We have used chronic unpredictable stress (CUS) to produce behavioral changes in rats that model several shared dimensions of PTSD and depression, including a deficit of cognitive flexibility, mediated in the medial prefrontal cortex (mPFC), and a shift from active to passive coping behavior, which models avoidant coping and withdrawal-related symptoms common to PTSD and depression. With this paradigm, we have demonstrated therapeutic effects using fear extinction learning as a novel rat model of exposure therapy. Extinction is an active learning process that modifies expectations of negative outcome predicted by cues previously paired with an adverse event. Similarly, exposure therapy, a form of cognitive behavioral therapy, is an active learning process that changes expectations of negative outcome predicted by cues previously associated with a stressful experience or context. We have shown that extinction training after CUS reverses the deficits in cognitive flexibility and coping behavior, tested 24 hr after treatment. We will now use this model to investigate neural mechanisms by which psychotherapy exerts its beneficial effect. Further, we will compare the mechanisms of extinction therapy with another therapy that has received recent attention, the NMDA receptor antagonist, ketamine. Ketamine has rapid antidepressant effects in treatment-resistant patients, and early studies in PTSD also show promise. However, ketamine has many undesirable qualities that limit its potential utility as a therapeutic agent. If mechanisms for its therapeutic effect can be identified, especially those shared by a non-pharmacological treatment, it may be possible to target those mechanisms separately from the ones responsible for its psychotomimetic and addictive properties. Thus, mechanisms shared by these two very different treatment modalities may represent potentially important new targets for the development of novel therapeutic strategies. Dysregulation of the mPFC is implicated in the pathology of PTSD and depression, and the mPFC is a critical component in the circuitry mediating extinction learning and cognitive flexibility. Thus we will focus our investigations on the mPFC. Activity-dependent protein synthesis is required for the long-term retention of extinction learning, and signaling pathways involved in activity-dependent protein synthesis are also activated by ketamine. Also, effects of extinction and ketamine are evident 24 hr or more after treatment, implying that neural plasticity is involved, as it is in learning. Thus, our overall hypothesis is that mechanisms underlying the therapeutic effects of extinction and ketamine involve activity-dependent protein synthesis in the medial prefrontal cortex, which initiates plasticity-related processes that enhance cognitive flexibility and active coping. Each component of this hypothesis will be tested in three specific aims: Aim 1 is to test the necessity of activity in the mPFC using a pharmacogenetic DREADD approach to selectively inhibit the mPFC during extinction or ketamine treatment. Aim 2 is to test the role of activity-dependent protein synthesis in the mPFC, by microinjecting selective inhibitors during treatment. We will first inhibit BDNF signaling, an activity-dependent neurotrophic factor that initiates a signaling cascade that induces protein synthesis and activates ribosomal protein S6, a component of protein translation. We will then test microinjection of the ribosomal protein synthesis inhibitor, anisomycin. Aim 3 is to examine functional and structural plasticity in the mPFC after extinction or ketamine treatment, measuring changes in the electrical response in mPFC to excitatory afferent stimulation, and changes in dendritic spine density. The results of this project will provide insight into the common and unique mechanisms underlying the beneficial therapeutic effects of two very different treatments for PTSD and depression, which may suggest new therapeutic targets.